In a pneumatic tire such as a radial tire mounted on a vehicle such as an automobile, a tread portion is formed in order to enhance a grip performance on a road surface. This tread portion is constituted in a manner that a plurality of rib grooves extended in a tire circumferential direction (grooves in the circumferential direction) are formed on a tread surface portion in contact with the road surface to form a plurality of rib lines, that a large number of lug grooves extended in a tire width direction (lateral grooves) are further formed on these rib lines at an appropriate interval, and that blocks (land portions) are formed by these rib grooves and lug grooves.
Incidentally, in order to reduce pattern noise caused by such a pattern of the tread portion, techniques such as changing a geometric relationship between the grooves and a shape of footprints are frequently used from a viewpoint that an impact causes the noise. The impact is caused in a manner that the pattern of the tread portion, and particularly, the blocks surrounded by the rib grooves and the lug grooves are brought into contact with the road surface.
However, among problematic phenomena in the noise caused by such a pattern of the tire, there is noise heard in a vehicle cabin during vehicle running (hereinafter, referred to as “pattern noise”).
This pattern noise as a phenomenon contains a component directly radiated from the tire. However, because a frequency of the noise is equal to or lower than 1000 Hz, it is conceived that the noise is also pretty much affected by an indirect sound generated in a manner that the tire vibrates an axle to vibrate a vehicle body.
It can be grasped that the indirect sound in this case is caused by variations of tire axle force, which result from the lug grooves becoming portions discontinuous in terms of rigidity in the tire circumferential direction. Specifically, at a moment when the lug grooves contact the road surface, spaces in the lug grooves become portions where the rigidity is lowered. Then, a load is lowered to a great extent, leading to great variations of the tire axle force.
In this connection, the present invention is one created in consideration of such a conventional problem. It is an object of the present invention to provide a pneumatic tire, in which, for variations of tire axle force, which are caused by portions discontinuous in terms of rigidity, such as lug grooves, reverse axle force is generated, and thus pattern noise resulting from the portions discontinuous in terms of the rigidity on a tread portion is reduced.